Space Invaders Game Instructions Set up the Environment Delete - - PowerPoint PPT Presentation

Space Invaders Game

Instructions

Set up the Environment

• Delete Scratchy Sprite (right click, delete)
• Create backdrop
• Select Backdrop from tabs
• Edit it
• Make it completely black
• If you want, add some stars (small white dots) to the backdrop
• Create new sprite for ship
• Import “ship” from the USB drive
• Create missile sprite
• Import “arrow” from the USB drive

Concepts

• Space Invaders has a lot of different objects on the screen
• The aliens
• The ship that shoots
• The missile that the ship shoots
• Each item has their own set of events
• It’s kind of like each has a mind of their own!
• When you want to make an object do something, make sure you are

creating events and actions in that specific sprite

Game Play – Events for your space ship

• Right Arrow
• Move 5 steps to the right
• Left Arrow
• Move 5 steps to the left

Space Ship Code

Game Play – Events for your arrow

• When Space Bar Clicked you want the arrow to shoot up the screen
• How do you do this?
• Create space key hit event for this sprite
• Repeat 40 times
• Change Y by 10
• This moves the arrow up the screen in increments of 10
• BUT
• You want the arrow to come out of the space ship so use a secret command
• Go to Ship!
• You could speed it up or slow it down
• How?

Arrow Code

Creating an Alien

• Import Sprite Invader1 from the USB drive
• Place on top left of screen

Moving the Alien

• Aliens move back and forth on their own so
• You need to programmatically determine their direction
• Create a variable called direction
• Start the Alien at position -201, 120
• Set direction= 10 (that’s 10 steps to the right)
• In a forever loop
• If the x position > 200, move the Alien down 25 spaces and change the

direction to -10 (that’s 10 steps to the left)

• Likewise, if the x position < 200, move the Alien down 25 spaces and change

the direction to 10 (that’s 10 steps to the right)

Moving the Alien

• When you run this, the Alien goes across the screen in a very smooth

way

• This will make shooting him hard so
• Let’s add a pause after he moves
• Command is wait
• Do it for .5 seconds
• You might want to also wait when you move the alien down a row

Moving the Alien - Code

Shooting the Alien – SCORE!

• When you shoot the arrow at the Alien, you should score a point
• The Alien should also die and disappear from the screen
• Steps
• Create a variable called score and initialize it to 0 when the game starts
• In the Alien sprite, create a script that is executed when the flag is clicked

(yes, you now have 2 flag clicked events)

• Check to see if the arrow and Alien are touching and if they are
• Incrementthe score by 1
• Hide the Alien sprite

Score! - Code

Add More Aliens!

• Duplicate or Clone the Aliens
• But
• You will need to move the new Aliens to a new starting position
• What X position (recommend they are 50 spaces apart and 25 spaces on the Y axis)
• Create a few rows
• What happens?

Personalize Your Game

• Want to stop here and perfect your game play?
• Do the aliens move the way you want them to move?
• If not, change it now
• Does the missile move fast (or slow) enough?
• If not, change it now
• Do you have enough aliens?
• Too close together?
• Too far apart?
• Make the game your own!

Taking Your Game to the Next Level

Advanced Concepts

Making Everyone Do the Same Thing at the Same Time…Broadcast

• When you put multiple rows of Aliens on your screen, they centipede

down the screen. We need our Aliens to all move at the same time!

• Do this using a Broadcast message
• Create a Broadcast called moveDown
• Instead of moving down and changing direction, simply call

moveDown

The moveDown Event

But Something is Wrong…

• The Aliens are going crazy on the screen!
• Why?
• Aliens do not change direction until they read the end of the screen
• So you still get crazy behavior
• How do you fix this?

Fixing the Aliens - Code

Problem 2 – Magic Bullets!

• If you have multiple rows of Aliens, one bullet seems to kill more than
• ne Alien. Let’s make the game harder.
• Once you hit an Alien, the bullet needs to stop
• Create a Broadcast called hitAlien and call it when an Alien is hit
• In the Arrow scripts, add a variable called stopFire. When this is set to 0, let

the bullet continue. When it is set to 1, the bullet will stop.

• When we receive the hitAlien Broadcast, set stopFire to 1
• In the space key event
• Show the Arrow
• Set stopFire to 0
• In the repeat, check if stopFire is 1. If it is, hide the bullet and stop this script.

Fixing the Magic Bullet - Code

Adding Barriers

• Let’s add some barriers so we can hide from Alien fire!
• A barrier will protect us from Alien fire (will absorb the missile) but

will also absorb fire from the ship

• Use Sprite barrier from the USB drive
• Place on screen
• Adjust arrow to sense the barriers
• If it is touching the arrow, stop the bullet, just like you did when you hit an

Alien!

Barriers - Code

What if the Alien Could Shoot?

• Let’s make the Aliens drop bombs
• If a bomb hits the ship, the game is over
• How to do this
• Import sprite alienFire from the USB drive
• The alienFire sprite will receive two broadcasts that you will create
• alienFire (when an Alien fires a missile – this happens every time it moves)
• stopAlienFire (when the missile hits something)
• The Barrier sprite will need to broadcast stopAlienFire when it touches the

bomb

• The Ship sprite needs to broadcast stopAlienFire when it touches the bomb

and it needs to end the game

Code Samples

More Code Samples

Ghost Aliens!

• We made it so one alien is shooting, but
• What happens if that alien is shot?
• He still shoots! He’s a ghost shooter!
• How do we fix this?
• Create a variable called alienVisible and set it to 1
• When the alien is shot (and hidden) change this to 0
• Before you fire, check alienVisible and ONLY shoot if it equals 1

The Code

Personalize It

• Which aliens should be shooting at the ship?
• Change this around if you want

Other Things to Try

• There are more graphics on the USB drive
• Want to change the alien’s appearance as they move across the screen?
• Add sounds
• Give your ship multiple lives and end the game once all lives have

been used

• Add more barriers